1. Field of the Invention
The present invention relates to a novel compound, a probe containing such a novel compound, and a contrast agent for fluorescence imaging, the contrast agent containing such a novel compound or such a probe.
2. Description of the Related Art
When a disease is treated, it is considerably important to recognize variations caused in a body (biological system) at an initial stage of the disease. In particular, visualization of the locations and the size of tumors by diagnostic imaging provides considerably useful data for diagnosing or treating the disease. Such visualization can be achieved by an existing technique such as fluorescence imaging, ultrasonic imaging, photoacoustic imaging, X-ray imaging, MRI (magnetic resonance imaging), CT imaging (computerized tomography imaging), or PET (positron emission tomography).
Fluorescence imaging is conducted by radiating near-infrared light having relatively high transmittance through biological tissues to a biological system to thereby excite a contrast agent that has been administered to the biological system in advance, and detecting fluorescence emission from the contrast agent in the biological system. Thus, visualization of the locations and the size of tumors can be achieved. Dyes that absorb light in the near-infrared region and emit fluorescence in the near-infrared region are used as such contrast agents for fluorescence imaging. Among such dyes, a cyanine dye has been prominently studied in recent years (WO2002/026891).
WO2002/026891 discloses a compound in which a molecule (capture molecule) selectively binding to a target site is fixed to a cyanine dye molecule with a peptide bond between the capture molecule and the cyanine dye molecule. However, such a cyanine dye compound has a problem of being slowly discharged from biological systems. Specifically, when a cyanine dye compound in which capture molecules are fixed with peptide bonds is administered to a biological system, an enzyme in the biological system may cleave the peptide bonds. As a result of such cleavage of the peptide bonds, cyanine dye molecules without the capture molecules become present in a large amount in portions other than the target sites. Thus, when the target sites are subjected to fluorescence imaging, fluorescence emission from such sole cyanine dye molecules becomes background emission, which can cause degradation of measurement sensitivity. Accordingly, there has been a demand for the development of a dye that is rapidly discharged from biological systems, that is, a dye that is cleared rapidly (having rapid clearance).